So, you think you may have a faulty fuel pump? Troubleshooting the problem is usually pretty straightforward. But occasionally, a tricky diagnosis will have you pulling out your hair and throwing wrenches. That’s why it’s helpful to have a few extra diagnostic tricks up your sleeve.
Before beginning, you’ll want to have the vehicle’s repair information handy. Manuals, such as those from Chilton, are good but an ALLDATA subscription is even better. ALLDATA has single-vehicle subscriptions for DIYers that provide detailed factory repair information.
Also, keep in mind, other issues can mimic a faulty fuel pump. You’ll want to perform a thorough diagnosis to ensure the pump is the problem before replacing it. And consult the previous article in this series regarding fuel pump symptoms to understand how both continuous and returnless fuel systems work.
Note: The following is for informational purposes only. Consult the factory information for repair instructions and recommended safety procedures.
The first step is to verify the problem is fuel related. Then, you can do additional testing to find out if the pump is to blame.
If you decide to attempt the test, try to start the vehicle by removing the air intake and spraying starter fluid into the throttle body. Have an assistant crank the engine while you spray – if it starts and runs momentarily, the problem is fuel-related and may indicate a bad pump. You’ll want to check fuel pressure as your next step.
If fuel trim is above 10, that means the engine is running lean and the fuel pump may not be delivering. Keep in mind, however, other factors, such as vacuum leaks, can cause a lead running condition as well. So, you’ll want to check fuel pressure as your next step.
Checking fuel pressure and/or volume will tell you whether or not enough fuel is making it from the tank to the engine. But that doesn’t necessarily mean the pump is bad. It’s wise to do further testing to rule out fuel pump circuit issues and other problems, such as a clogged fuel filter or bad pressure regulator, that could limit the fuel supply.
Caution: Before hooking up a fuel pressure gauge, be sure to relieve fuel system pressure as outlined in your repair manual. Have your safety glasses on and a fire extinguisher nearby.
Performing a static fuel pressure test is pretty straightforward. Simply connect the gauge as outlined in your repair manual. Most modern vehicles have a handy test port located on the fuel rail.
Next, turn the ignition to the on position (key on, engine off), note the reading on the gauge and compare it to the specification listed in your repair manual. If the reading varies significantly from spec, you may be dealing with a bad fuel pump.
Also, when performing a static test, you’ll want to note whether the pressure holds. The factory repair information will tell you how long (usually several minutes) fuel pressure should hold once the pump is turned off. If pressure bleeds off too fast, there is a fuel-related problem such as a bad pump check valve or faulty regulator.
Those looking to perform a more in-depth fuel pressure assessment may choose to check fuel pressure at idle or under load. Also, continuous fuel systems can be checked for “dead head” pressure (pressure with the return line pinched off). But that test does not work on newer returnless systems that have the regulator in the tank.
Start by connecting the gauge and placing the bleed end of its hose into a clean container. Without starting the engine, activate the pump for 15 seconds while you hold down the relief valve on the tool. Typically, the pump should deliver about a pint of fuel during that time. Consult the repair information for the exact procedure and specifications for your vehicle.
So, you’ve verified a lack of fuel pressure and/or volume and now, you think you can condemn the pump. But wait – not so fast. In many cases, you’ll want to check the following beforehand:
In many newer, returnless fuel systems, the fuel pump control module (FPCM) directly controls the fuel pump. The device uses information from sensors, such as the fuel pressure sensor and fuel temperature sensor, to determine control of the pump’s ground circuit and vary the pump’s speed. It’s a good idea to make sure everything within the control system is working properly before blaming the pump.
With both continuous and returnless systems, you’ll also want to check the integrity of the wiring in the pump circuit before jumping to conclusions. A digital multimeter (DMM) can be used to check the circuit for continuity, power and ground, as well as any unwanted excessive resistance.
There are a couple of advanced tests available for those who have the interest – and the money – to try them out.
One of the best ways to test a fuel pump is with an oscilloscope and low amp clamp probe. Connecting the probe to the pump’s power wire provides a waveform pattern that, if you know what you’re looking for, exposes internal pump problems. Generally, a good pump should produce a pattern of even “humps” if it’s working properly. If the pump is bad, the humps will be jagged and uneven.
A missing wave in the pattern usually indicates a burned-out winding or an open circuit. Meanwhile, waves that vary in shape and size point to worn brushes and commutators.
Before doing a fuel pump test using an oscilloscope, familiarize yourself with your vehicle’s connections and wirings. You’ll have access to the fuel pump relay to connect the digital oscilloscope.
An oscilloscope tracks how voltage signals change over time, displaying them on a screen. It’s like a digital graphing tool that shows different waveforms.
Aside from diagnosing automotive issues using voltage signals, an oscilloscope is also used in various applications in areas like electronics, telecommunications, and engineering.
Some high-end scan tools allow remote fuel pump testing. By pushing a couple of buttons on the tool, you can activate the pump to see if it runs.
Driving around with a faulty fuel pump is a bad idea. It can cause poor fuel efficiency, stalling, and engine damage. If you don’t want to pay a bigger repair bill, then you should visit a mechanic or replace your bad fuel pump as soon as possible. Luckily, getting a new fuel pump is fast and easy with CarParts.com.
CarParts.com offers a wide selection of fuel pumps that are all sourced from the most trusted manufacturers in the industry. For easier browsing, make sure to use our vehicle and search filters, so you can see all the available parts that are compatible with your ride. You’ll be pleased to know that CarParts.com also guarantees fast shipping. Thanks to our strategically located warehouses around the US, you can expect your new fuel pump to arrive in as fast as two business days. Just be sure to order by 12 PM ET.
Don’t wait for your faulty fuel pump to damage other parts of your engine before replacing it. Check out our catalog of high-quality fuel pumps at CarParts.com and order today!
Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic. The accuracy and timeliness of the information may change from the time of publication.